The invention relates to a new economic process for recovering and purifying paraxylene from a charge of hydrocarbons which contain paraxylene at a concentration that is greater than that at thermodynamic equilibrium. It relates, in particular, to the processes that enrich their effluent with paraxylene to more than 30%, particularly the processes that provide at least one paraxylene-enrichment step by crystallization at very low temperature, for example, this crystallization step being followed by a step of purifying the paraxylene in at least one stage (U.S. Pat. Nos. 3,177,265 and 2,866,833).
The invention relates in particular to a process for preparing and purifying paraxylene from a charge of aromatic hydrocarbons having 8 carbon atoms comprising a combination of steps for selective adsorption, for purification by high-temperature crystallization and for isomerization as described in the patents of the applicant FR 2,681,066 and U.S. Pat. No. 5,284,992 that are incorporated by reference.
It applies in particular to the preparation of paraxylene with very high purity for the production of terephthalic acid for the synthesis of synthetic fabrics.
Crystallization has been used commercially for a very long time to isolate and purify paraxylene, typically from a mixture of xylenes and ethylbenzene close to equilibrium (C8-aromatic fraction).
The C8-aromatic fractions ordinarily cone from a reforming unit or from an ethylene production unit and especially from the reformates by distillation alone or extraction in combination with a distillation as a function or the composition of the charge, of the sensitivity to impurities of the downstream technologies and of the savings of the recovery processes.
The typical composition by weight of a C8-aromatic fraction is approximately 22% paraxylene, 16% ethylbenzene, 18% orthoxylene and 44% metaxylene. Very low temperatures are generally required to effectively recover, by crystallization, the paraxylene from a C8 fraction. Furthermore, there is a eutectic limit that prevents the complete recovery of all the paraxylene from a C8 fraction. For example, in a low-temperature crystallization unit, for a C8-aromatic fraction containing 22% weight of paraxylene, only about 50 to 65% of the paraxylene is recovered, the remaining paraxylene will be found in the paraxylene-depleted mother liquor, which can be introduced into an isomerization unit. The latter will isomerize the metaxylene, orthoxylene and in certain processes, the ethylbenzene, and a mixture of xylenes close to equilibrium containing about 22% paraxylene will again be obtained. This recycled flow, in combination with the fresh charge, is then introduced into the crystallizer so as to recover more paraxylene. In this way, the C8-aromatics can be recycled to extinction and recovery of a maximum amount of paraxylene, with by-products resulting from the isomerization. The production of these by-products is a significant insufficiency of the system since each tire that xylenes are introduced into the isomerization unit, a part of them is converted into non-xylenes such as toluene. Actually, the chemistry of the isomerization is very complex and the main reactions which lead to losses of xylenes are the disproportionation (dismutation) of the xylenes into toluene and trimethylbenzenes, dealkylation or the xylenes, and in certain cases even the formation of non-aromatics. To do this, the total yields of a set of isomerization and crystallization units are typically 60 to 80% and a large isomerization recycling loop is necessary to maximize the recovery of the paraxylene.
In the 1970""s, another process was marketed to prevent the eutectic limitation from a low-temperature crystallization. This process uses adsorption to separate the paraxylene form a mixture of xylenes. The adsorption makes it possible to recover more paraxylene from a C8-aromatic fraction. Thus, from a charge containing 22% weight of paraxylene, it is possible to recover approximately 97% of the latter by adsorption, leaving about 1% paraxylene in the mother liquor. It is advantageous to recover the product with a greater efficiency, since this entails the use of a much smaller isomerization loop for the complete recycling of the paraxylene-depleted fraction. This offers several advantages, particularly a lower investment cost on a new unit or on the expansion of an existing unit, overall higher yields due to low losses in isomerization, and lower operating costs linked to the size of the isomerization loop.
Several drawbacks linked to the system for recovery by adsorption of paraxylene are noted, however: high investment cost, difficulty in obtaining paraxylene with very high purity, sensitivity of the adsorbent to impurities in the charge and sensitivity of the control system to the changes in quality of the charge.
Furthermore, a process for crystallization that uses two separate crystallization stages (AMOCO process) has been proposed. Demand for a higher purity of paraxylene was increasingly difficult to satisfy a single crystallization. A process was therefore developed which completely remelts the crystals that were isolated from the first crystallization stage at very low temperature. After complete remelting, the flow was cooled to about xe2x88x9217xc2x0 C. to recrystallize the paraxylene to the desired purity. This process was able to deliver high purities (99.5%+) after washing. However, it has the drawback of higher operating and investment costs due to the cost of the energy associated with a complete remelting and then to a recrystallization of the paraxylene.
A process of the applicant has recently been patented, which combines in particular an adsorption with a crystallization; it is patent U.S. Pat. No. 5,284,992 which describes a selective adsorption to recover the paraxylene from a charge containing a mixture of xylenes. The paraxylene is then purified by at least a high-temperature crystallization. It is taught that there is a synergy between the adsorption and the high-temperature crystallization, due to the fact that the adsorption is a very efficient process for recovering paraxylene from xylenes and that the crystallization is a very efficient means for purifying paraxylene to a very high level, which constitutes an ideal link between the two technologies. Moreover, this process emphasizes the full advantage of a smaller isomerization loop due to an almost quantitative recovery by pass of the paraxylene into the adsorption step of the unit.
Another patent U.S. Pat. No. 5,329,060 teaches a process comprising a selective adsorption step of a charge of a mixture of xylenes followed by a double-stage crystallization of paraxylene, one at very low temperature (xe2x88x9250 to xe2x88x9270xc2x0 C.) and the other at high temperature (0 to xe2x88x9210xc2x0 C.). The operating and investment cost of this process is higher since it provides a crystallization at very low temperature and a complete melting of the crystals obtained before their recrystallization.
Moreover, in the sequence of steps comprising an adsorption, a crystallization and especially an isomerization, to obtain very pure paraxylene, various types of impurities can appear in the various effluents and cause disturbance in the operation of units thus interfering with the yield obtained and the purity of the paraxylene recovered.
First, during isomerization of the paraxylene-depleted fraction, olefinic hydrocarbons can be produced in a variable amount depending on the values of the partial pressures of hydrogen introduced. The subsequent formation of polymers before and/or in the adsorption unit can cause serious problems with circulation through the adsorbent, and even destroy the adsorbent. Moreover, paraffinic and naphthenic hydrocarbons with 8 and 9 carbon atoms, whose volatility is between that of a desorption solvent, for example toluene, and that of the xylenes, are intermediate products of the conversion of ethylbenzene into xylenes during the isomerization and their accumulation can prove to be harmful. Furthermore, aromatic hydrocarbons with 9 carbon atoms and more, present in small proportion and poorly separated in distillation columns, can be detrimental to the process, just like aldehydes and ketones that are heavier than the initial charge, which are formed when oxygen is accidentally dissolved.
Finally, another problem is linked to the presence of methanol. This alcohol is at times added in small proportion in mixtures of xylenes to be crystallized to prevent the co-crystallization of water and of paraxylene. Actually, the mixtures of dry C8-aromatics are particularly hygroscopic and when the suspension of paraxylene crystals in the mother liquor passes into the centrifuge, water contained in the ambient air can be absorbed in the mother liquor and this water can possibly crystallize in connection with the temperature of this mother liquor. Moreover, some exchangers can have leaks and water can pass accidentally into the mixture to be crystallized.
An object of the invention is to produce paraxylene with the highest possible purity with a greater flexibility and at the most economical cost possible.
Another object is to eliminate the drawbacks mentioned.
A further object is to limit the amount of the impurities, particularly in the adsorption section, to try to optimize it, to the extent that the adsorbent is very sensitive to the impurities of the charge of the adsorption zone.
It has therefore been observed that by using a high-temperature crystallization in a single stage, or advantageously a high-temperature crystallization in several stages and preferably in two stages followed by a partial melting of the recovered crystals, a process was obtained with a better recovery of paraxylene and also very economical especially since the cooling fluids used are easy to use. Furthermore, the risk of increasing the concentration of undesirable impurities in the effluents is minimized.
More specifically, the invention relates to a process for the production of paraxylene of very high purity from a charge containing a mixture of aromatic hydrocarbons having 7 to 9 carbon atoms in which at least a part of the charge is circulated in a zone suited to enrich a first fraction to at least 30% by weight of paraxylene, and at least a portion of said first fraction is purified by at least one high-temperature crystallization in at least one crystallization zone, the process being characterized in that:
a) said first fraction enriched with paraxylene is crystallized in a crystallization zone at high temperature T1 and advantageously, between +10 and xe2x88x9225xc2x0 C.,
b) crystals in suspension are recovered in a mother liquor,
c) the crystals of the mother liquor are separated in at least a first separation zone, preferably at a temperature that is approximately constant and approximately equal to that of crystallization T1,
d) the crystals of step (a) are partially melted in at least a partial melting zone and a suspension of crystals is recovered;
e) the crystals in suspension of step (d) are separated and washed with a suitable washing solvent in at least one zone for separating and washing, and, on the one hand, pure paraxylene crystals, and, on the other hand, a washing liquor are recovered; and
f) said pure crystals are optionally completely melted and a liquid stream of melted paraxylene is collected.
The washing step can include pure paraxylene liquid product.
By high-temperature crystallization is meant a crystallization in at least one crystallizer for each crystallization stage of a solution or suspension of paraxylene, already enriched with paraxylene, which corresponds to what the literature calls a purification step. For example, U.S. Pat. No. 2,866,833, incorporated as a reference, mentions a step of purifying paraxylene at high temperature able to go up to a temperature of xe2x88x9234xc2x0 C.
By carrying out a step of partial melting of the crystals, the surface impurities are in part eliminated, the crystals of small size are melted, and the temperature of the crystals is increased which makes possible an efficient operation of the means for separating and for washing the paraxylene crystals mentioned below, and consequently for achieving very high purities. The heat input necessary for the partial melting can be performed in the partial melting zone itself and/or upstream from it, thanks, for example, to the recycling of at least a portion of the optionally heated washing liquor.
According to a first variant, the enrichment zone of the first fraction with at least 30% by weight of paraxylene can be at least one crystallization zone at very low temperature, for example, lower than xe2x88x9240xc2x0 C., said recovery section, such as the one described in U.S. Pat. No. 2,866,833 or such as those described in U.S. Pat. No. 5,329,061 that are incorporated by reference and in which a charge containing aromatic hydrocarbons having 8 carbon atoms is introduced. This enrichment zone delivers a suspension of crystals which is separated in a separation zone and the recovered crystals are melted and constitute at least a part of said first fraction to be purified later on. Moreover, a mother liquor resulting from the separation can be isomerized in an isomerization zone and the isomerate at least in part recycled to the enrichment zone (recovery section).
According to a second variant, the paraxylene-enrichment zone can be a selective adsorption zone containing a zeolitic adsorbent and in which a charge containing aromatic hydrocarbons having 8 carbon atoms is introduced. A selective adsorption of the charge is carried out, in the presence of a desorption solvent, said first paraxylene-enriched fraction and a second paraxylene-depleted fraction are recovered, said second fraction is isomerized in an isomerization zone containing a catalyst for isomerization under conditions suitable for producing an isomerate containing paraxylene approximately at thermodynamic equilibrium with isomers and the isomerate is recycled at least in part to the adsorption zone.
The desorption solvent is generally selected as a function of the nature of the adsorption. By way of example, it is possible to use toluene, paradiethylbenzene, difluorobenzene, diethyl toluene, or an alkyltetraline, particularly described in U.S. Pat. Nos. 4,886,929, 4,864,069 and 5,057,643 which are incorporated by reference.
According to a third variant the paraxylene enrichment zone can be a disproportionation zone of a charge consisting essentially of toluene and using a coke-selectivated catalyst or a silicon-selectivated catalyst according to U.S. Pat. Nos. 4,117,026, 4,097,543, 4,851,604, 5,173,461, 5,243,117 and 5,321,183 incorporated by reference.
The unreacted toluene and the benzene are advantageously removed by distillation from the disproportionation effluent comprising the xylenes.
It is advantageous to have an effluent containing more than 50% by weight of paraxylene and preferably 75 to 98% coming out of the enrichment zone, i.e., for example of the selective adsorption zone as described in U.S. Pat. No. 5,284,992 of the applicant.
At least a portion of the mother liquor resulting from a separation step after the one-stage high-temperature crystallization of the charge of hydrocarbons can be recycled to the enrichment zone, for example to the adsorption zone. This separation step can be carried out by at least one centrifuge or at least one rotary filter, which are means known to one skilled in the art.
In the same way, the step of separating and washing crystals, which is carried out in the same zone, after the step of partial melting, can be performed in at least one centrifuge or one rotary filter. According to a preferred variant, it can be carried out in a zone for separating and washing where the washing solvent is introduced countercurrent to the paraxylene crystals to be washed as described in U.S. Pat. Nos. 4,475,355, 4,481,169 and CH 515,730 that are incorporated by reference. More specifically, this zone for separating and washing can comprise at least one washing column, such as a NIRO column.
The resulting washing liquor, optionally distilled if necessary, can be advantageously recycled at least in part in the crystallization zone, a portion being able to be sent to the partial melting zone to keep the proportion of crystals at a suitable level.
However, when the zone for separating and washing is a centrifuge, it is also possible to recycle a portion of the washing liquor in the separation zone coming out of the crystallization stage, which makes it possible to have a better purity of the final paraxylene.
According to another particularly advantageous embodiment making it possible to reduce the operating cost of the process, it is possible to perform the purification of the paraxylene in at least two crystallization stages at high temperature T1 and T2 and preferably between +10 and xe2x88x9225xc2x0 C. In this case, steps a, b, c and d described above are carried out, at least a portion of the mother liquor resulting from step c) is crystallized in a second crystallization zone at a high temperature T2 that is lower than temperature T1 of the crystallization zone of step a), second crystals in suspension are recovered in a second mother liquor, said second crystals of said second mother liquor are separated in a second separation zone, the second crystals are melted partially in at least one zone for partial melting, a suspension of said first crystals resulting from step d) and second crystals are recovered, the crystals obtained are separated and washed in at least one zone for separating and washing with the washing solvent, pure paraxylene crystals, on the one hand, and a washing liquor, on the other hand, are recovered, and optionally said pure crystals are completely melted, then the liquid stream of melted paraxylene is collected.
It is possible to recycle at least a portion of the washing liquor in the first crystallization zone, another portion being able to be recycled to the partial melting zone, which recovers the crystals of two crystallization zones so as to maintain a concentration of crystals of about 35% by weight, for example, in this tank.
As has been stated above, the step of separating and washing the crystals after the step of partial melting can be carried out either in at least one countercurrent washing column, or in at least one centrifuge or one rotary filter. All of the crystals in suspension can be separated and washed in the same element or else the first crystals in suspension can be separated and washed in at least one column, one centrifuge or one rotary filter and the second crystals in suspension can be separated and washed in at least one separate column, one separate centrifuge or one separate rotary filter. In the case of washing by at least one centrifuge or one rotary filter, it is possible to recycle a portion of the washing liquor in the first separation zone following the first crystallization stage and optionally a portion of the washing liquor in the second separation zone coming out of the second crystallization stage.
According to a characteristic of the process preferably comprising two crystallization stages, at least a portion of the mother liquor coming out of the second separation zone can be recycled to the enrichment zone, and more specifically to the selective adsorption zone.
It can be particularly advantageous to maintain a certain amount, about 30% by weight, for example, of crystals in the first crystallization effluent and in the second crystallization effluent. For this purpose, it is possible to recycle a portion of the first mother liquor in the first crystallization zone and a portion of the second mother liquor in the second crystallization zone. The temperature and the proportion of crystals in each crystallizer thus are controlled independently.
It has been observed that by operating at a first crystallization temperature T1 advantageously between +10 and xe2x88x925xc2x0 C., and preferably between +5 and xe2x88x921xc2x0 C., and at a second crystallization temperature T2 between xe2x88x925 and xe2x88x9225xc2x0 C. and preferably between xe2x88x929 and xe2x88x9225xc2x0 C., excellent results are obtained, the temperature selected depending on the type of cooling agent, on the optimization between the enrichment steps, for example, by adsorption, the crystallizations and the final washing.
According to a characteristic of the process, the final washing of the paraxylene at the end of the process, carried out in at least one centrifuge or one rotary filter or in at least one countercurrent washing column, a NIRO column for example, can be performed by a portion of the liquid stream or pure paraxylene coming from the complete melting step, which is used as washing solvent.
According to another characteristic of the process, the final washing of the paraxylene at the end of the process, carried out in at least one centrifuge or one rotary filter or in at least one countercurrent washing column, a NIRO column for example, can be performed by a washing solvent which is a solvent other than paraxylene, toluene, hexane or pentane for example. In this case, the stream of melted paraxylene is distilled, there is recovered, on the one hand, paraxylene of very high purity, and, on the other hand, washing solvent that is recycled at least in part in the zone for separating and washing, the washing liquor that contains washing solvent in a small amount is also distilled, and at least a portion of the washing solvent that is distilled in this way is recycled in the zone for separating and washing.
When the operation is with a countercurrent washing column, and a solvent other than paraxylene, it can be advantageous to separate the suspension of paraxylene crystals coming from this column by a filter or a centrifuge, before the melting step of the paraxylene and to recycle the solvent separated in this way in the washing column.
The washing liquor, after distillation, is then generally recycled as indicated above.
The partial melting zone is generally effected at a temperature between the temperature of the coldest crystallization stage of the purification zone and the melting temperature of the pure paraxylene, and preferably between 0 and 11xc2x0 C. to melt, for example, between 5 and 60% by weight of the crystals.
It generally comprises a tank receiving the crystals and heating means, for example by steam.
It is possible to use a single zone for partial melting, which recovers the first and second crystals.
However, it can prove preferable to melt partially the first and the second crystals which have a different degree of purity in two separate zones. In this case, the recycling of the washing liquor can be envisioned in each of the two zones to maintain the proportion of crystals at a suitable level.
The process according to the invention makes it possible to obtain paraxylene of very high purity.
It has been observed, however, that it was possible to improve the performances of the equipment by controlling at all levels the proportion of impurities that can disturb the selective adsorption of the paraxylene on the absorbent and its crystallization, whether at the level of the charge of aromatic hydrocarbons, the isomerate and/or the recycled mother liquor. (Patent application of the applicant Ser. No. 94/15,896.)
More generally, it is possible to circulate a portion selected from at least in part the charge, at least in part the mother liquor, at least in part the isomerate in at least one treatment zone with clay or equivalent material, and a first effluent is recovered that is introduced at least in part in the adsorption zone or in the very low temperature crystallization zone.
According to the variant comprising a purification with a crystallization stage, it is possible to circulate respectively the mother liquor resulting from the separation step in at least one clay treatment zone before recycling it in the enrichment zone. In particular in the adsorption zone or in the very low temperature crystallization zone.
According to the variant comprising a purification with two crystallization stages, the second mother liquor resulting from the separation of the crystals coming out of the second crystallization zone is introduced into the clay treatment reactor.
These clay treatments make it possible to eliminate at least in part the olefins created in particular in the isomerization step and at least a portion of the heavy impurities, which circulate in the enrichment zone, crystallization and isomerization loop.
Various variants can be envisioned:
The mother liquor can be advantageously introduced at least in part into a distillation column, advantageously that downstream from the isomerization zone. This column also treats the effluent of the isomerization zone and delivers a top fraction containing light compounds (water, methanol, C7-) and. another fraction containing a distilled mixture of mother liquor and isomerate that is then introduced into the clay treatment zone.
A distillation bottom fraction containing heavy compounds can also be drawn off from this distillation column, which makes it possible to reduce the size of the downstream equipment.
A portion of the mother liquor can also be mixed with the effluent, whatever it is, leaving the clay treatment zone, whether this is the effluent resulting from the circulation or the isomerate, the mother liquor or the charge in the clay treatment zone, or the effluent resulting from the circulation, in the clay treatment zone, of the latter and of the distillation fraction containing said distilled mixture of mother liquor and isomerate, before being introduced into the selective adsorption zone.
The resulting effluent of these latter variants can be distilled in at least one distilling column (a so-called rerun column) which delivers a bottom fraction containing heavy compounds and a top fraction which is introduced into the adsorption zone optionally with a portion of the mother liquor.
The conditions for adsorbing or eliminating undesirable compounds in the clay are, as a general rule, the following:
Temperature: 100 to 300xc2x0 C., preferably 160 to 230xc2x0 C.
Hourly space velocity: 1 to 8, preferably 1 to 4 (Hourly volume of charge per volume of clay)
Type of clay; activated natural aluminosilicates, for example, the clay referenced F54 with ENGELHARD.
Pressure: 3 to 100 bar, preferably 4 to 20 bar.
The distillation column, depending on the isomerization, generally has the following characteristics:
Pressure: 1 to 20 bar, preferably 3 to 8 bar
Base temperature; 150 to 280xc2x0 C., preferably 200 to 240xc2x0 C.
Number of plates: 30 to 80, preferably 50 to 70.
The distilling column, known as rerun, located between the clay treatment zone and the selective adsorption zone usually has the following characteristics:
Pressure: 1 to 20 bar, preferably 3 to 8 bar
bottom temperature: 160 to 290xc2x0 C., preferably 210 to 250xc2x0 C.
Number of plates: 40 to 200, most often 50 to 90.
According to another characteristic of the invention, it is possible to keep the amount of constituents with intermediate volatility between that of the desorption solvent and that of paraxylene, at a tolerable level. In this case, at least a portion of the mother liquor can be purged before being introduced into the clay treatment zone.
It can also be advantageous to purge at least in part the desorption solvent resulting from distillation steps of the depleted fraction or of the paraxylene-enriched fraction before it is recycled and to compensate for the purge of the solvent by an addition of fresh solvent, either into the charge or upstream from the adsorption zone, for example.
As has been indicated, it is possible to recycle the crystallization mother liquor in different places of the installation depending on the magnitude of the amounts of undesirable compounds, but it can be advantageous to combine these different recyclings with one another, for example, when it involves reusing existing pieces of equipment for the distillation of the isomerate, the clay treatment or the so-called rerun distillation and when one of these pieces of equipment is already operated at its maximum flow rate.
It is also possible to combine these different recyclings and these purges when it is desired to have the amount of an impurity in the loop reduced without seeking to eliminate it altogether.